1. Field
The present invention relates, in general, to a multi-channel Light-Emitting Diode (LED) driving device and, more particularly, to a multi-channel LED driving device which is driven in a linear driving manner.
2. Description of the Related Art
Generally, an existing LED has been widely used as a backlight unit for a liquid crystal display device used in mobile phones, Personal Digital Assistants (PDAs), or notebook computers. Further, with the development of LED manufacturing technologies, efficiency is increased, brightness is greatly improved, and thus LEDs are not only used as light sources for large-sized LCD devices, such as televisions (TVs), but also widely used for typical lighting, security lights, street lamps, etc. An LED has a long lifespan, environment-friendly characteristics, and an expectation to be widely used for normal lighting in the future via continuous efforts to improve optical efficiency.
Generally, an LED is driven using a current driving scheme, and uses a commercial power of AC 220V or 110V when the LED is used for normal lighting. Further, driving schemes may be chiefly divided into a converter scheme in which an inductor and a capacitor are used as in the case of a switching mode power supply (SMPS), and a linear scheme in which an SMPS is not used. In the case of the converter scheme, electrical efficiency and optical efficiency are higher than those of the linear scheme, but the configuration of the system is complicated, and a large amount of noise appears when switching is performed, thus resulting in electromagnetic interference (EMI) and electromagnetic compatibility (EMC). Further, in the case of the converter scheme, a separate power factor correction circuit must be used to improve the power factor, and an additional circuit for suppressing the occurrence of electromagnetic waves upon switching must be configured, and thus the configuration of the system is complicated and the cost thereof is high. A typical linear scheme has a simple system configuration, but has low electrical efficiency and a low power factor, and thus it is not widely used. An improved linear scheme has been introduced so as to solve this disadvantage and extensive efforts have been made to improve the power factor and efficiency, but there is still much room for improvement in this scheme that has been introduced.
FIG. 20 illustrates a conventional LED driving circuit based on an initial linear driving scheme. This is a structure for driving LEDs by simultaneously turning on/off all the LEDs in an interval during which an input voltage is higher than a voltage required to turn on all the LEDs, and exhibits the characteristics of a low power factor and low efficiency, but enables a simple configuration.
FIG. 21 illustrates a conventional LED driving circuit based on an improved linear driving scheme. This is configured to apply a scheme for dividing LEDs into 3 to 4 channels and sequentially driving LED channels in response to an input voltage, thus improving the power factor and efficiency. However, this scheme is limited in that an interval during which each channel is operated must be previously set by sensing an input voltage, and in that when the voltage of an LED is changed within a preset voltage range, reduced efficiency or deteriorated characteristics are evident, thus making it very difficult to configure multiple channels. Further, the configuration of a circuit for multi-channel configuration is also very complicated. When the number of channels increases, efficiency and power factor, which are the most important characteristics of lighting, can be simultaneously improved.
In the case of 4-channel driving, a single channel has a voltage drop of about 60V based on an AC input voltage of 220V. If schematic efficiency is calculated via the number of operating channels for each input voltage and an average voltage, the following Table 1 may be obtained.
TABLE 1LED voltage drop(V)/Intervalnumber ofaverageIntervalVin (V)operating channelsvoltage (V)efficiency (%)Interval 1 0-60 0/030—Interval 2 60-120 60/19066.7Interval 3120-180120/215080.0Interval 4180-240180/321085.7Interval 5240-311240/4275.587.1
For schematic calculation, overall efficiency is calculated by averaging the efficiency values of respective intervals, and is 79.9%. When only interval 5 is used without sequentially turning on LEDs, electrical efficiency is high, but an interval during which LEDs are turned on is short, and thus optical efficiency is low and the power factor is also low.
In the case of 8-channel driving, if a voltage drop of one channel is assumed to be 35 V, and efficiency is calculated in the same manner, the following Table 2 is obtained.
TABLE 2LED voltage drop(V)/Intervalnumber ofaverageIntervalVin (V)operating channelsvoltage (V)efficiency (%)Interval 1 0-35 0/035Interval 235-70 35/152.566.7Interval 3 70-105 70/287.580.0Interval 4105-140105/3122.585.7Interval 5140-175140/4157.588.9Interval 6175-210175192.590.9Interval 7210-245210227.592.3Interval 8245-280245262.593.3Interval 9280-311280295.594.6
In this way, in the case of 8-channel driving, the overall efficiency is 86.8%, which exhibits an increment of 6.9% compared to 79.9% in the case of 4-channel driving. However, since the input voltage has the form of a sine wave, and the slope thereof in intervals 1 and 2 is much sharper than that in intervals 8 and 9, a percentage occupied by intervals 1 and 2 is very low from the standpoint of time. If the overall efficiency is calculated again with the exception of intervals 1 and 2, it is 90.9%, which appears greater.
However, as shown in FIG. 21, when it is desired to detect an input voltage, select an area in which each LED group is turned on, and increase the number of LED groups (identical to the number of channels of a driving unit), the range of voltages at which each LED is operated is narrowed, and thus it is very difficult to set the operating range depending on variations in the voltage of LEDs. Further, a variation in the voltage drop of LEDs depending on temperature acts as a very considerable obstacle to an increase in the number of channels. In addition, as the number of channels increases, the size of a block required to detect the input voltage increases. As a result, most lighting systems are configured using only LED groups of 3-4 channels.